1. Field of the Invention
The present invention pertains to circuitry for sorting objects according to the color thereof, and more particularly, it pertains to circuitry for sorting fruit according to color by separately measuring the light reflected from the surface of a fruit within two different bands of wavelengths of light and comparing the same.
2. Description of the Prior Art
Colorimetry, i.e., the analysis of objects upon the basis of their color, has many industrial applications, particularly in the paint and dye industries, and complex and sophisticated circuits have been devised for accurately determining various shades of color. Generally speaking, attempts to apply the methods and sorting circuitry of these industries to the sorting of fruits and vegetables have been unsuccessful for a number of reasons. In matching paints and dyes, one set of measurements often suffices to establish the color of an entire batch. Even in continuous process control, changes in the color of the paints and dyes are usually gradual, and circuitry which can adjust itself rapidly to large changes in color is not required. In fruit and vegetable sorting, on the other hand, the color of each article must be separately determined, usually in a small fraction of a second, and successive determinations may lie at the extremes of the range of measurement.
The matching of paints or dyes of different composition requires a knowledge of their reflectance properties throughout the visible spectrum. A system of trichromatic coefficients is used to describe their variance in reflectance in the simplest possible terms. Logically then, the color of paints and dyes is usually measured in terms of these coefficients. Whereas this system is desirable for the comparison of different combinations of pigments, it is needlessly cumbersome for the color classification of any one fruit or vegetable. For example, it is not usually necessary to distinguish a yellow lemon from a green apple. It may, rather, be safely assumed that any one peculiarity in the reflectance properties of an apple of a given color will be characteristic of all apples of the same color. Hence, color measurements of fruit and vegetables can usually be confined to only one or two regions of the spectrum wherein such anomalies are known to occur.
Great precision is required in the matching of paints for the eye is able to distinguish small variations in color. Precise determination of the color of fruits and vegetables is seldom justified. Even if apples could readily be sorted into twenty-five color classes, it would scarcely be practical to market this number of grades. Furthermore, the variation in color over the surface of a piece of fruit is usually so great as to make a precise color measurement meaningless, and, in accordance with usual fruit color sorting practices, grading is performed upon the basis of the percentage of the "characteristic" color on the surface of the fruit.
Color sorting circuitry which has been specifically designed for the sorting of fruits and vegetables generally provides some means for measuring the reflectance properties of the fruit or vegetable being tested. The reflectance of a surface is a measurement of the percentage of incident light reflected by it, and colored objects have different reflectances for light of different wavelengths. The relationship between reflectance and the illuminating wavelength for a fruit being tested will produce a characteristic curve which can then be used in the design of apparatus and circuitry for color rating that fruit. That is to say, a fruit may be classified as to color by suitably measuring, describing, and classifying its reflectance curve, and fruit may be sorted into different grades by denoting the differences between the reflectance curves for the various grades and testing for these differences. The efficacy of such a system depends to a great extent upon the nature of the particular criterion used to describe and characterize the reflectance curves.
Several criteria of color similarity have been investigated in the past, and circuitry has been developed for their measurement. None of these circuits have proven to be wholly successful, however, in the high speed sorting of fruit and vegetables. The simplest prior art method of classifying a fruit as to color was circuitry which characterized the reflectance curve for the fruit by a single measurement of reflectance. Obviously, this measurement was made in the region of the spectrum where the change in reflectance between consecutive color grades was greatest. In color grading Washington delicious apples, for example, measurements of the reflectance would be made at a wavelength of approximately 560 nanometers wherein the variation in reflectance between the color grades is greatest. To measure this reflectance, the fruit was illuminated with a light restricted to a narrow band of wavelengths in the vicinity of 560 nanometers, or the fruit was illuminated with light of a wide band of wavelengths with an optical filter being used to receive the reflected light so as to restrict the transmitted light to a narrow band of wavelengths in the vicinity of 560 nanometers. The reflected light was directed to a photodetector, and the resulting photoelectric current was proportional to the reflectance of the fruit. The problem with such methods of making color determinations is that the measured reflectance not only varies with the color of the fruit but also varies with the intensity of illumination, the photodetector sensitivity, the fruit size, and the location and orientation of the fruit with respect to the light source and the photodetector. These latter factors usually made the reflectance measurements unreliable and led to errors in color grading.
An improvement over the aforedescribed circuits is provided by circuitry which measures the reflectance in two bands of wavelengths of light rather than in just one band. One of the selected bands will include a wavelength wherein the variation of reflectance between distinct color grades is at a maximum, and the other band will comprise wavelengths wherein there is little or no variation in reflectance between the different color grades of fruit. The determination of the color of a fruit can then be measured by observing the difference in the value of the reflectance at the two different bands of wavelengths. While such a system is more sensitive to color variations than the aforedescribed circuitry, this circuitry was still primarily dependent upon the total amount of light reflected from the surface of the fruit which total light varied due to a variety of factors and none of which were directly related to the color of the fruit.
A still further method of determining fruit color, wherein the measured value is largely independent of the total amount of light received from a fruit being inspected, has been used in certain color sorting apparatus. This method utilizes the aforedescribed method of measuring the reflectance properties of a fruit at two distinct wavelengths; however, rather than merely computing the difference between the two measurements, the ratio of these two measurements is computed so as to eliminate the errors due to variations in the total amount of light reflected because of factors other than color. While such a method is generally used in the trichromatic color measuring devices of the paint and dye industries and has been adapted in a few instances in fruit and vegetable color sorters, the circuitry which has been designed to carry out such a method has proven to be exceedingly complex and expensive and, therefore, not readily adaptable to the fruit and vegetable packing industry wherein competition with the human fruit sorter is keen. Examples of fruit sorting circuitry which utilized such a method of sorting, or variations thereof, include the circuitry shown in the prior U.S. Pat. Nos. to Powers 2,933,613, Cox 3,012,666, and Cox 2,244,826.
Another prior art color sorting apparatus is disclosed in the patent to Roberts et al 3,206,022. The circuitry disclosed in this patent utilized a ratio monitoring system wherein reflectance values at two selected wavelengths of light were measured. A predetermined percentage of one measurement was then compared with the other measurement on a "zero monitoring" system, or differential basis, whereby a series of such comparisons based upon predetermined fixed ratios established the limits of the tested reflectance ratio. This system eliminated the errors due to varying intensity in the light received from the fruit because of factors other than color (typically, the size of the fruit), although the system did not obtain a true reflectance ratio reading. While the circuitry was not as complex as the true ratio detection systems and represented a compromise between the ratio detection systems and the simpler circuitry of the prior art, it still was complex enough that it required frequent servicing and high initial cost. Furthermore, adjustments were difficult to make, and the circuitry was not readily adaptable to sorting different types or varieties of fruit. The performance of the apparatus utilizing the circuitry shown in the Roberts et al patent did not represent a sufficient increase in sorting capacity to overcome the inertia of the conservative and skeptical fruit packing industry. .Iadd.
Another prior art sorting apparatus is disclosed in the patent to Mustert 3,679,314. The apparatus disclosed in this patent uses alternating light beams of different spectral intensity distribution to provide output signals which are compared by means of a divider that forms a ratio of signals. An evaluating circuit makes an acceptance decision if the ratio is within given tolerance limits. The patent does not show or suggest the circuitry necessary to form the ratio of signals or to make the acceptance decision. The apparatus shown in Mustert is used to test the genuineness of bank notes which are in a fixed position near a source of light. .Iaddend.